DESCRIPTION

The recvfrom() and recvmsg() system calls are used to receive messages
from a socket, and may be used to receive data on a socket whether or not
it is connection-oriented.
If from is not a null pointer and the socket is not connection-oriented,
the source address of the message is filled in. The fromlen argument is
a value-result argument, initialized to the size of the buffer associated
with from, and modified on return to indicate the actual size of the
address stored there.
The recv() function is normally used only on a connected socket (see
connect(2)) and is identical to recvfrom() with a null pointer passed as
its from argument.
All three routines return the length of the message on successful
completion. If a message is too long to fit in the supplied buffer,
excess bytes may be discarded depending on the type of socket the message
is received from (see socket(2)).
If no messages are available at the socket, the receive call waits for a
message to arrive, unless the socket is non-blocking (see fcntl(2)) in
which case the value -1 is returned and the global variable errno is set
to EAGAIN. The receive calls normally return any data available, up to
the requested amount, rather than waiting for receipt of the full amount
requested; this behavior is affected by the socket-level options
SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2).
The select(2) system call may be used to determine when more data
arrives.
The flags argument to a recv() function is formed by or'ing one or more
of the values:
MSG_OOB process out-of-band data
MSG_PEEK peek at incoming message
MSG_WAITALL wait for full request or error
MSG_DONTWAIT do not block
The MSG_OOB flag requests receipt of out-of-band data that would not be
received in the normal data stream. Some protocols place expedited data
at the head of the normal data queue, and thus this flag cannot be used
with such protocols. The MSG_PEEK flag causes the receive operation to
return data from the beginning of the receive queue without removing that
data from the queue. Thus, a subsequent receive call will return the
same data. The MSG_WAITALL flag requests that the operation block until
the full request is satisfied. However, the call may still return less
data than requested if a signal is caught, an error or disconnect occurs,
or the next data to be received is of a different type than that
returned. The MSG_DONTWAIT flag requests the call to return when it
would block otherwise. If no data is available, errno is set to EAGAIN.
This flag is not available in strict ANSI or C99 compilation mode.
The recvmsg() system call uses a msghdr structure to minimize the number
of directly supplied arguments. This structure has the following form,
as defined in <sys/socket.h>:
struct msghdr {
void *msg_name; /* optional address */
socklen_t msg_namelen; /* size of address */
struct iovec *msg_iov; /* scatter/gather array */
int msg_iovlen; /* # elements in msg_iov */
void *msg_control; /* ancillary data, see below */
socklen_t msg_controllen;/* ancillary data buffer len */
int msg_flags; /* flags on received message */
};
Here msg_name and msg_namelen specify the destination address if the
socket is unconnected; msg_name may be given as a null pointer if no
names are desired or required. The msg_iov and msg_iovlen arguments
describe scatter gather locations, as discussed in read(2). The
msg_control argument, which has length msg_controllen, points to a buffer
for other protocol control related messages or other miscellaneous
ancillary data. The messages are of the form:
struct cmsghdr {
socklen_t cmsg_len; /* data byte count, including hdr */
int cmsg_level; /* originating protocol */
int cmsg_type; /* protocol-specific type */
/* followed by
u_char cmsg_data[]; */
};
As an example, one could use this to learn of changes in the data-stream
in XNS/SPP, or in ISO, to obtain user-connection-request data by
requesting a recvmsg() with no data buffer provided immediately after an
accept() system call.
Open file descriptors are now passed as ancillary data for AF_UNIX domain
sockets, with cmsg_level set to SOL_SOCKET and cmsg_type set to
SCM_RIGHTS.
Process credentials can also be passed as ancillary data for AF_UNIX
domain sockets using a cmsg_type of SCM_CREDS. In this case, cmsg_data
should be a structure of type cmsgcred, which is defined in
<sys/socket.h> as follows:
struct cmsgcred {
pid_t cmcred_pid; /* PID of sending process */
uid_t cmcred_uid; /* real UID of sending process */
uid_t cmcred_euid; /* effective UID of sending process */
gid_t cmcred_gid; /* real GID of sending process */
short cmcred_ngroups; /* number or groups */
gid_t cmcred_groups[CMGROUP_MAX]; /* groups */
};
The kernel will fill in the credential information of the sending process
and deliver it to the receiver.
The msg_flags field is set on return according to the message received.
MSG_EOR indicates end-of-record; the data returned completed a record
(generally used with sockets of type SOCK_SEQPACKET). MSG_TRUNC
indicates that the trailing portion of a datagram was discarded because
the datagram was larger than the buffer supplied. MSG_CTRUNC indicates
that some control data were discarded due to lack of space in the buffer
for ancillary data. MSG_OOB is returned to indicate that expedited or
out-of-band data were received.

RETURNVALUES

These calls return the number of bytes received, or -1 if an error
occurred.

ERRORS

The calls fail if:
[EBADF] The argument s is an invalid descriptor.
[ECONNRESET] The remote socket end is forcibly closed.
[ENOTCONN] The socket is associated with a connection-oriented
protocol and has not been connected (see connect(2)
and accept(2)).
[ENOTSOCK] The argument s does not refer to a socket.
[EMSGSIZE] The recvmsg() system call was used to receive rights
(file descriptors) that were in flight on the
connection. However, the receiving program did not
have enough free file descriptor slots to accept them.
In this case the descriptors are closed, any pending
data can be returned by another call to recvmsg().
[EAGAIN] The socket is marked non-blocking, and the receive
operation would block, or a receive timeout had been
set, and the timeout expired before data were
received.
[EINTR] The receive was interrupted by delivery of a signal
before any data were available.
[EFAULT] The receive buffer pointer(s) point outside the
process's address space.